1. Field of the Invention
The present invention relates to a digital switching amplifier for outputting digital switching signals to drive a resistive, capacitive, or inductive load such as a loudspeaker, headphone, piezoelectric element, heating element, electric motor, or for outputting electrical signals such as voltage, current, or charge.
2. Description of the Related Art
In an audio field, for instance, a digital switching amplifier (or D class amplifier) has been known as an amplifier for driving a low resistive load such as loudspeaker, headphone, and the like to produce an analog audio signal. The amplifier is very useful as a component used in a miniaturized set because it produces a large amplitude output and can be easily integrated into LSI due to its lower heating characteristic resulting from small voltage drop in an amplifier.
In such a field, however, there have been several problems to be solved before manufacturing it as an excellent amplifier. The most important problems which have been tackled include, in particular, improvements in signal-to-noise ratio (S/N ratio) at the time of a small signal output, decrease in current consumption also at the time of a small signal output, reduction in electromagnetic interference (hereinafter referred to as “EMI”).
A digital switching amplifier disclosed in JP 2000-49613A has been known as a first example of the related art thereof. This amplifier switch-outputs a noise-shaped Pulse Density Modulation (PDM) signal with plural-value levels. An example of the circuit is shown in FIG. 14.
As shown in FIG. 14, the first example of the related art includes a delta sigma modulating section consisting of a differential integrator 102 and quantizer 103, delay element 104, clock oscillator 105, pulse amplifier 106, low pass filter 107, and attenuator 109.
The delta sigma modulating section delta-sigma-modulates an analog input signal 101, converting into a digital signal. The quantizer 103 converts the analog input signal 101 into the positive/negative digital signals with four values or more and even values. The pulse amplifier 106 switch-controls the application of constant voltage to a load 108 via the low pass filter 107 by the switching control signal corresponding to the positive/negative digital signals with four values or more and even values.
The first example of the related art can ensure a higher S/N ratio because quantizing noises can be decreased as compared to a conventional digital switching amplifier called one-bit amplifier for switch-outputting a PDM signal with binary levels. The application of a lower voltage pulse to a load always (that is, at all times) at the times of a small signal output and of no signal output can further reduce current consumption in comparison with the one-bit amplifier which always applies a higher voltage pulse to a load.
In other words, the current consumption of the one-bit amplifier is VDD/R, where R is a load resistance and VDD is voltage applied across a load. On the other hand, in the amplifier for outputting four-valued levels for each polarity, for example, if a voltage applied across a load at the times of small signal output and of no signal output is 0.25×VDD which is provisionally prepared and is the smallest level, the current consumption equals (0.25×VDD)/R, being one fourth of that of the one-bit amplifier.
Another digital switching amplifier disclosed in JP2003-101357A has been known as a second example of the related art thereof. This amplifier switch-outputs a noise-shaped Pulse Width Modulation (PWM) signal with binary levels. An example of the circuit is shown in FIG. 15.
As shown in FIG. 15, the second example of the related art includes an integrating circuit 201 for integrating an input signal 200, a flash A/D converter 202 for A/D converting the output signal of the integrating circuit 201, a waveform conversion circuit 203 for generating a PWM signal of which pulse width corresponding to a digital value outputted from the flash A/D converter, a switching circuit 204 consisting of a pair of MOS transistors 205 and 206 connected between a first power supply VDD and a second power supply VSS, a node P of the pair of MOS transistors 205 and 206 being connected to a loudspeaker 208 serving as a load through a low pass filter 207, a driving circuit 209 for driving the pair of MOS transistors 205 and 206 in response to the PWM signal outputted from the waveform conversion circuit 203, and a feedback resistor RNF serving as a feedback circuit which is connected to the node P and the input side of the integrating circuit 201 and negatively feeds back an output signal from the amplifier being supplied to the loudspeaker (load) 208.
The second example of the related art constituted in the above manner can further reduce the number of switchings than the foregoing digital switching amplifier called one-bit amplifier for switching and outputting a PDM signal with binary levels, so that EMI can be decreased. The application of an electrical signal to a load only a period during which a pulse width modulation signal is active can reduce an time ratio of an active pulse applied to a load in the times of small signal output and no signal output, thereby reducing a current consumption.
In other words, in the second example of the related art, the current consumption is given by (0.25×VDD)/R, where R is a load resistance, time width of the provisionally prepared smallest pulse for the period of PWM conversion is 0.25, and VDD is voltage applied to a load. This is one fourth of that of the one-bit amplifier using a PDM signal.
The present inventors have not been satisfied with the above related art and therefore endeavored to further increase an S/N ratio at the time of small signal output, further reduce a current consumption at the time of small signal output, and further decrease EMI, as a result, the present inventors have invented a digital switching amplifier far superior to the related art.
That is, the purpose of the present invention is to provide a digital switching amplifier capable of further increasing an S/N ratio and lessening a current consumption at the time of a small signal output and decreasing EMI.